The present invention relates to load sharing between two electric energy sources, such as a DC to AC inverter on the one hand and the utility grid on the other hand, and in particular, such as a DC to AC converter deriving its DC energy input from solar panels or a solar-charged battery
In the prior art, two types of DC to AC converters for use in solar energy installations are known.
One type of inverter, called a grid-tie inverter, converts DC energy to an AC current and feeds the current back into the electricity grid to reduce net energy consumption. Such an inverter is said to operate in “current mode” as the voltage is defined by the utility grid to which it is connected, and it is controlled to output a current at that voltage, the product of which is the desired power level to be transferred from the DC source to the grid.
The big disadvantage of this method is that, if the utility is in an outage, the system must be switched off. Another disadvantage is that, as the penetration of solar energy increases, there will come a point where the grid cannot accept all the aggregated power that solar systems are attempting to feed back into it on a sunny day. Other disadvantages include political issues such as the requirement for connection permission from the utility, permitting from local or State authorities, utility subversion of solar via unattractive billing regimes and barriers to innovation such as the National Electrical Code and Listing requirements.
A second type of inverter, called a standalone inverter, converts DC power to an AC voltage equivalent to the grid voltage which can be used to power loads directly, thereby avoiding consuming power from the grid. Energy storage, i.e. a battery, is required for standalone systems to smooth out the difference between supply and demand as clouds move over the sun uncorrelated with appliance loads being switched on and off.
U.S. Pat. No. 8,937,822 to Applicant Dent, entitled “Solar Energy Conversion and Utilization System”, discloses both grid-tie and standalone DC to AC inverters, and discloses an adaptively controlled distribution panel that selects which breaker circuits to power from a standalone inverter and which to power from the electric utility grid, thereby assisting in dynamically matching supply and demand.
When it is attempted to utilize a standalone inverter to power a home in a totally off-grid installation, a very large and expensive battery is required to ensure the ability to bridge a period of overcast weather. However, the above '822 patent discloses a system that uses both grid and solar power and adaptively transfers load between utility and solar in such a way that the net solar power used in place of utility power matches the average solar power received over a shorter period of time, thus allowing the battery size and cost to be drastically reduced.
Another method of using a current-mode inverter that is known in the prior art is called “utility assist mode”. This is substantially the same as grid-tie mode, except that the inverter current is controlled to supplement current coming from the grid without ever feeding power back to the grid. The current is thus controlled to be less or equal to the current consumed from the grid by active appliances. The advantage of this method is that a finer degree of matching between solar power generated and solar power used can be achieved. However, since the inverter is still in parallel with the grid, it must switch off if the grid is in outage, unless a fast disconnect switch operates and simultaneously switches to voltage-mode DC to AC inversion. For the latter, a battery will also be required for energy smoothing. It is also not clear that connection permission is not required, as the inverter current waveform is being impressed on the grid and must meet utility specifications and all of the requirements of specification UL1741. There is therefore a need for an alternative method of ensuring the use of just as much solar power as is momentarily available while avoiding ever connecting the inverter to the grid. The requirement may be summarized in general as a method and apparatus for continuously sharing load between two energy sources without a direct electrical connection between the two energy sources.